This invention relates to an orbital wastewater treatment system. This invention also relates to an associated method of operating an orbital wastewater treatment system.
In a wastewater process employing an activated sludge process, wastewater impurities including domestic wastes, sugars, lipids, proteins, carbohydrates and other nitrogen- and phosphorous-containing materials are decomposable by microorganisms, as is well known in the art. As the impurities are decomposed, a sludge of settled material and microorganisms is wasted from the process either on a continuous or non-continuous basis. The purpose of sludge wasting is to keep solids from building up in the system. Sludge from the process is normally transported by pumping to a digester for treatment prior to landfill or other disposal so as to reduce the volatile organic content of the sludge, reduce the sludge volume, reduce the pathogenic organisms present in the sludge, reduce its odor potential and improve sludge dewaterability, and for other reasons of lesser importance. Various prior art types of digesters and various digestion and stabilization processes have been proposed and used.
An early circa 1960 installation by Pasveer for wastewater, i.e. primarily sewage, purification by the activated biological sludge method included a closed circuit or ditch with a horizontally-rotated brush rotor used for adding needed oxygen (air) to the sewage and moving the sewage in circulation. In U.S. Pat. No. 3,510,110, an orbital system employing an elongated tank with central partition was disclosed which employed a vertically-rotated surface aerator located at the end(s) of the partition wall for both aerating the sewage and circulating the sewage around the channels formed by the partition wall and the sides of the tank. This latter system has had great commercial success with over 1000 plants in operation world-wide (ranging from less than 1 MGD capacity to one of over 10 million population equivalent) with over 600 plants in operation or in various construction phases since 1976 in the United States up to the present time. Sold under the trademark “Carrousel®”, the high popularity of the system is due primarily to its cost-effectiveness, simplicity of design, ease of operation and maintenance, and excellent effluent quality. It can treat raw domestic water to EPA advanced secondary standards without primary clarifiers or effluent filters. With extended aeration, it produces a highly stable water sludge requiring little or no further processing prior to disposal. Carrousel® orbital wastewater treatment systems can be designed to have a power turn-down of 50 to 85 or 90%. Aerator drive horsepower can be varied from 100% of installed capacity to as little as 10% without loss of mixing and continuing sufficient mixed liquor channel velocity. (See U.S. Pat. No. 4,869,818.) This power turn-down flexibility provides an ability to closely match oxygen input to the mixed liquor to oxygen demand of the microbes acting to degrade the sewage, without loss of mixing and movement. In one of the largest U.S. installations, over 25 MGD of sewage is treated in four units having twelve aerators utilized to aerate and circulate sewage through sixteen channels formed by twelve partitions and exterior encircling concrete walls forming four basins.
Improvements in Carrousel® orbital wastewater treatment systems are disclosed in U.S. Pat. No. 4,869,818, U.S. Pat. No. 4,940,545 and U.S. Pat. No. 7,186,332. In general, each Carrousel® orbital wastewater treatment system sold under the trademark denitIR® includes a tank having at least one partition that defines an anoxic zone and an aerobic zone that are operated in accordance with the modified Ludzack-Ettinger (MLE) Process. The partition also defines passages from said aerobic zone to said anoxic zone and from said anoxic zone to said aerobic zone. At least one impeller/aerator is located in said tank for moving mixed liquor under process about said tank and for increasing the dissolved oxygen content of the liquor in the aerobic zone. An flow-diversion gate is provided at the passage for controlling the recycling of nitrates to the anoxic zone from the aerobic zone. (See U.S. Pat. Nos. 8,318,016 and 8,857,674.) The aerator is efficient in oxygen transfer and mixing so as to maintain solids in suspension while varying oxygen input so that the main channel flow reaches an anoxic condition as it passes the flow-diversion gate. In the anoxic basin or zone, screened and degritted influent and recycled activated sludge are mixed with nitrified mixed liquor, providing optimized conditions for high rate denitrification, pursuant to the MLE Process. Bacteria feed on the carbon-rich influent, using molecular oxygen from the abundant nitrate to drive metabolic reactions. Nitrate is first reduced to nitrite, then to nitrogen gas, which is subsequently stripped in the aeration basin. In the process, portions of the alkalinity and oxygen consumed during nitrification are restored.
A low-speed surface aerator in orbital wastewater treatment tanks typically takes the form of a specially designed impeller spinning over a range of about 15 to 50 rpm. The impeller has a fixed number of blades, which pump water from below and throw off a large spray pattern of small droplets. This process provides dissolved oxygen to the water. The combination of aerator speed (as controlled, for instance, by a variable frequency drive) and aerator submergence (as controlled by an adjustable weir that adjusts water level relative to the aerator blades) determines the horsepower draw of the unit. The higher the horsepower, the higher the mass of dissolved oxygen in the water. Low-speed surface aerator transfer efficiencies are generally expressed as pounds of dissolved oxygen delivered to the wastewater per unit horsepower per hour (lb O2/HP-hr) reported under standardized conditions of atmospheric pressure, temperature and other variables.
There are essentially an infinite number of combinations of aerator submergence and speed that draw the same horsepower (higher sped and lower submergence vs. lower speed and higher submergence). While aerator transfer efficiencies (lb/HP-hr) are generally reported as a constant value, the actual efficiency varies with aerator diameter, basin geometry, aerator submergence, and aerator speed. Through decades of testing, no one has been known to find a method to accurately and consistently predict aerator transfer efficiencies as a function of these and other variables. The industry typically designs systems and reports transfer efficiencies based on average or minimum values derived from actual testing.
If, once an aerator impeller diameter and basin geometry have been selected, the aerator could be operated at the conditions of submergence and speed that produce an enhanced, if not maximized, transfer efficiency, significant energy savings would result. These savings could be on the order of 10-20%. Given that aeration is a large consumer of power (typically 40-60% of a wastewater treatment plant's energy bill), this would be a feature of interest to municipalities.
Orbital wastewater treatment plants optionally include a forced-air diffuser. Thus, in some plants, aeration of the wastewater may be accomplished by both mechanical means—impeller aeration—and by diffused aeration.